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FILE – In this Dec. 9, 2015 file photo, crews work on a relief well at the Aliso Canyon facility above the Porter Ranch area of Los Angeles. The escape of tons of natural gas from under a Los Angeles neighborhood is taking months to stop because of pressure from the leak. The leak at Porter Ranch started in October, and likely won’t be fixed for at least two more months. Officials have relocated several thousand residents who said the stench made them sick. (Dean Musgrove/Los Angeles Daily News via AP, Pool, File)

State regulators are investigating whether the controversial drilling practice of hydraulic fracturing, or fracking, contributed to the massive natural gas leak near Porter Ranch.

Fracking at Aliso Canyon had not been widely reported, though it is common at California’s underground gas storage facilities.

More than two months after Southern California Gas Co. detected a leak at its Aliso Canyon field, observers are searching for reasons the well may have failed. Some environmentalists are drawing attention to fracking, while experts caution that such a rupture is unlikely.

The leaking well’s maintenance records don’t indicate that it was fracked, according to a review of the file released by the state Division of Oil, Gas & Geothermal Resources. But at least one nearby well in Aliso Canyon was fracked, the records show.

The process releases gas by directing pressurized liquid and fracturing rock. Called “well stimulation,” fracking is used by natural gas operators to increase production from underground reserves.

A state-commissioned report found that Aliso Canyon was a major producer of hydraulically fractured gas, compared to California’s other natural gas storage facilities. Collectively, about a third of the gas stored in these state reservoirs is derived from fracking, according to the 2015 report by the California Council on Science and Technology.

“Obviously, fracking these old wells raises some real concerns about dangers to well integrity — fracking can cause casing to fail,” said Patrick Sullivan, spokesman for the Center for Biological Diversity.

But rupturing a well casing because of high-pressure fracking is unusual, according to Paul Bommer, senior lecturer in the Department of Petroleum Engineering at the University of Texas, Austin. And it would only affect the well that was fracked, he said.

Another concern would be one well’s fracking-produced rock fissures could cause another well casing to rupture. That might happen if the fracking and the leak were at the same depth, Bommer said, but it would be “very unlikely” to affect another well at a shallower depth.

State officials have indicated well SS-25 could be leaking around 500 feet below ground. The nearby well, SS 4-0, was fracked at deeper than 9,000 feet.

How deep the fracking occurred is key, said Gene Nelson, a physical sciences professor at Cuesta College in San Luis Obisbo. The process could be “very benign,” he said, if it was limited to the natural gas reservoir, and not deeper.

Environmentalists and state officials aren’t ready to rule it out as a possible cause yet.

“In fracking, you can’t control exactly where the fissures are going to go,” said Kyle Ferrar western program coordinator for the environmental group FracTracker Alliance. “It’s a possible cause, but it’s impossible to know if it happened.”

In their investigation plan, state officials say they will examine well records, including those pertaining to “well stimulation operations.”

“These are the kinds of questions that will be examined as a part of the division’s root cause analysis,” Department of Conservation Chief Deputy Jason Marshall said in an email.

A representative from the gas company did not respond to a request for comment on the fracking issue, but said, “We won’t speculate on the cause of the leak.”